The invention relates to an apparatus and a process for the application of a flowable medium from a stock chamber to a surface moved along the apparatus, where the stock chamber partly covers the surface with formation of a sealing gap and an exit gap. The invention furthermore relates to the use of an apparatus of said type.
Apparatuses of said type are widely employed, for example, in the production of labels for coating paper or film webs with adhesives. A common process for this purpose is the engraved roll application process, in which the flowable adhesive is located in an open stock chamber whose opening is in contact with a rotating roll. The roll contains a multiplicity of engraved grooves. During rotation of the roll, the initially empty grooves enter the opening region of the stock chamber from the outside and are filled with adhesive therein. On exit of the roll from the opening region of the stock chamber, excess adhesive is wiped off the surface of the roll by means of a doctor blade which extends over the width of the roll and limits the opening region of the stock chamber. During further rotation of the roll, the latter is brought into contact with the paper or film web to be coated, causing at least some of the adhesive in and on the grooves to be transferred to the web.
The amount of adhesive transferred from the stock chamber to the roll can be varied within narrow limits through the position or contact pressure of the doctor blade at the exit of the roll from the opening region of the stock chamber, i.e. ultimately through the height of the exit gap formed by the doctor blade and the roll surface.
A further doctor blade is generally installed in the region of the entry of the roll into the opening region of the stock container, this further doctor blade taking on the function of a seal between the stock container and the roll surface. In order to prevent excess outflow of the adhesive at this point, the sealing gap formed between this doctor blade and the roll surface must have the smallest possible height.
The contact pressure of the doctor blade at the sealing gap and the height of the exit gap can be adjusted in this known process by changing the position and situation of the application apparatus relative to the rotating roll.
In particular at high roll rotation speeds, complete filling of the roll grooves is no longer guaranteed in this known process, which results in the amount of adhesive taken up during a rotation and thus ultimately the application weight of the adhesive to the web to be coated dropping in an undesired manner. Furthermore, air is increasingly introduced into the stock container through the roll grooves at higher rotation speeds, resulting in undesired foaming in the stock container. The presence of foam in the stock container in turn means that the grooves of the engraved roll are not completely filled with adhesive, but instead partly with air in the form of bubbles, which means that fine bubbles form on the web to be coated, resulting in undesired clouding of the adhesive layer.
In order to prevent said difficulties, it has been proposed to pressurize the adhesive in the stock chamber using suitable means (J. Txc3xcrk, H. Fietzek, H. Hesser and I. Voges, Perspektiven fxc3xcr die Verarbeitung von Dispersionshaftklebstoffen, reprint TI/ED 1654d, BASF Ludwigshafen, August 1993). This ensures complete filling of the engraved grooves, even at high roll rotation speeds. Depending on the set pressure, a different amount of adhesive is also conveyed out of the application apparatus on the surface of the roll outside the engraved grooves at the exit gap. In this way, the amount of adhesive applied to the roll and thus ultimately the application weight of the adhesive on the web to be coated can be set within a broader range than without the use of pressure. Due to the higher pressure in the stock chamber, it is furthermore achieved that only a greatly reduced amount of air is introduced into the stock container at the sealing gap; in this way, excess foaming is prevented.
However, the higher the roll rotation speed, the higher the pressure in the stock chamber has to be selected in order to prevent the introduction of air into the adhesive. The maximum achievable rotation speed is limited by the fact that, on a further increase in pressure, the adhesive is forced out of the stock container in an uncontrolled manner firstly through the sealing gap and secondly through the exit gap. Exit of adhesive through the sealing gap results in undesired presentation of adhesive in front of this gap, which can result in soiling of the environment of the application apparatus and in operational interruptions. Uncontrolled exit of adhesive through the exit gap in turn results in uneven application of composition to the web to be coated.
In the article xe2x80x9cRasterwalzenauftragsverfahren mit Druckkammerrakelxe2x80x94ein Beschichtungswerkzeug auch fxc3xcr strahlenchemisch hxc3xa4rtende Systemexe2x80x9d, IPW 1/97, pp. 1 to 8, it was proposed to construct an application apparatus in such a way that the adhesive flows constantly through the stock chamber in the opposite direction to the direction of movement of the roll. In this way, air introduced into the stock container is constantly transported away by the rotating roll with the flowing adhesive. It was also proposed in this article to incorporate a throttle element into the stock chamber in such a way that a narrow throttle gap with a length of a few centimeters forms between the throttle element and the roll surface, through which gap the adhesive flows in the opposite direction to the direction of movement of the roll surface. In this way, the engravings are filled better with adhesive, and any air present in the engraved grooves is forced out of them, at least partly, and removed from the roll surface with the flowing-off adhesive. In practice, however, precise adjustment of the throttle element independently of the operating parameters, for example roll rotation speed, viscosity and pressure of the adhesive, proves to be difficult and not easily reproducible. In addition, an apparatus of this type cannot reliably prevent the introduction of air into the stock chamber and the formation of bubbles in the adhesive film.
It is an object of the present invention to provide an apparatus for the application of a flowable medium to a surface moved along the apparatus which works reliably even at high surface speeds, in which formation of air bubbles in the medium is reliably prevented, which is of simple construction, and which can be set simply and reproducibly for given operating conditions.
We have found that this object is achieved by an apparatus for the application of a flowable medium from a stock chamber to a surface being moved along the apparatus, where the application chamber at least partly covers the surface. In accordance with the invention, the stock chamber is divided into a pre-chamber and a main chamber, between which is arranged a dividing element which, together with the surface, limits a dividing gap.
The dividing element arranged between the pre-chamber and the main chamber reliably prevents air which has entered the pre-chamber from being transported into the main chamber and resulting in undesired foaming therein. The actual coating of the surface with the desired amount of the flowable medium then takes place free from air bubbles in the main chamber. The pre-chamber and the main chamber advantageously have independent feeds for the medium, so that, for example, the pressure conditions in the pre-chamber and in the main chamber can be selected independently of one another.
Particular advantages arise if the dividing element is arranged in such a way that it touches the surface. This achieves complete separation of the pre-chamber and the main chamber.
In practice, complete touching or contact of the dividing element with the surface can often only be achieved if the surface is stationary. In actual operation, i.e. in the case of a moving surface, unavoidable variations in the guidance of the surface mean that a certain, albeit very small gap height will usually be present. Particularly effective prevention of air bubbles and uniform application of the medium to the surface is also achieved if the dividing element is arranged in such a way that the height of the dividing gap between the dividing element and the surface is between 0 and 0.1 mm, in particular between 0 and 0.08 mm, preferably between 0 and 0.05 mm, particularly preferably between 0 and 0.02 mm. A minimization of the height of the dividing gap in this way likewise achieves effective separation of the pre-chamber and the main chamber.
In order to ensure reliable function of the apparatus under various operating conditions, it is advantageous for the dividing element to be arranged in a movable manner. Movement can consist both in a change in the situation of the dividing element, for example a tilting, a movement in or against the direction of movement of the surface or a movement toward or away from the surface, i.e. a change in the height of the dividing gap. Operating conditions which influence the optimum position and situation of the dividing element can be, for example, the nature and speed of movement of the surface, the pressure of the medium in the pre-chamber, the pressure of the medium in the main chamber, and the composition and viscosity of the medium to be applied.
Simple implementation of the separation of the pre-chamber and the main chamber arises if the dividing element contains a doctor blade. However, other embodiments of the dividing element are also conceivable. Particularly effective separation can be achieved if the dividing element is a double doctor blade. An essential prerequisite for the choice of a suitable dividing element is its sealing function separating the pre-chamber from the main chamber.
In an advantageous embodiment, the dividing element contains a cylindrical rod. This facilitates reliable and low-wear sealing between the pre-chamber and the main chamber.
Effective sealing can also be achieved if the dividing element contains a flexible leaf which is arranged in such a way that at least one edge of the leaf touches the surface in a resilient manner.
Particularly effective sealing between the pre-chamber and the main chamber can advantageously be achieved by designing and arranging the dividing element in such a way that it, together with the moving surface, limits at least two dividing gaps, where the height of each dividing gap is between 0 and 0.1 mm, in particular between 0 and 0.08 mm, preferably between 0 and 0.05 mm, particularly preferably between 0 and 0.02 mm. In a particularly preferred embodiment, the dividing element is arranged in such a way that the height of each dividing gap is 0 mm, i.e. the dividing element touches the moving surface.
The division of the stock chamber into a pre-chamber and a main chamber enables individual operating parameters to be modified separately for the pre-chamber and for the main chamber. In an advantageous embodiment of the invention, means are present with which the pressure of the medium in the main chamber can be set independently of the pressure of the medium in the pre-chamber. For example, the pressure in the main chamber can then be chosen to be higher than the pressure in the pre-chamber. This firstly avoids the medium being forced out of the pre-chamber in an uncontrolled manner through the sealing gap which seals off the pre-chamber from the outside, and secondly the increased pressure in the main chamber forces any air bubbles which pass through the dividing gap back into the pre-chamber.
Inverse pressure conditions in the pre-chamber and in the main chamber can offer particular advantages. In an advantageous process for operating the apparatus according to the invention, the pressure of the medium in the pre-chamber is higher than the pressure of the medium in the main chamber. In this way, penetration of air into the pre-chamber through the sealing gap can be effectively prevented from the outset. The pressure in the main chamber can then be varied within broad limits without the risk of foaming, in order to guarantee optimum layer application to the surface.
Further advantages arise if a throttle gap is additionally arranged between the pre-chamber and the main chamber. The throttle gap can be located between the pre-chamber and the dividing element or between the dividing element and the main chamber. The height of the throttle gap is always larger than that of the dividing gap. A throttle gap of this type can, for example, be formed by a region of the wall between the pre-chamber and the main chamber which is designed in such a way that it runs parallel to the moving surface at a small distance therefrom. It is furthermore conceivable for a special throttle element to be installed on the wall between the pre-chamber and the main chamber. The dividing element limiting the dividing gap can in this case be attached to the throttle element. The throttle element can be arranged so as to be adjustable in position and/or situation. In this way, the height and shape of the throttle gap can be changed. The length of the throttle gap can vary within broad limits. In particular, a conceivable throttle gap is one whose length is a multiple of the length of the dividing gap. The throttle gap can be directly adjacent to the dividing gap, but can also be arranged spatially separated therefrom. An important action of the throttle gap is to reduce the pressure difference between the pre-chamber and the main chamber. In this case, the throttle gap fulfils a sealing function supporting the function of the dividing gap. If, for example, the pressure in the main chamber is chosen to be higher than the pressure in the pre-chamber, a pressure gradient forms in the throttle gap, with the pressure dropping from the main chamber to the pre-chamber. Any air bubbles which have penetrated from the pre-chamber into the throttle gap are in this way additionally held back in the throttle gap.
Further advantages arise if means are present which allow the temperature of the medium in the pre-chamber, in the main chamber or in both chambers to be set independently of the ambient temperature. Thus, for example, the temperature in both chambers can be chosen to be significantly higher than the ambient temperature. This enables firstly the viscosity of the medium to be reduced and thus its flow properties to be improved, and secondly a drying process following application of the medium to the surface is accelerated.
Particular advantages arise if the temperature of the medium in the main chamber is selected to be higher than that of the medium in the pre-chamber. This reduces the viscosity of the medium in the main chamber compared with the viscosity of the medium in the pre-chamber, which facilitates penetration of small amounts of the medium from the main chamber into the pre-chamber through the dividing gap, but makes transport of the mediumxe2x80x94and thus also of any air bubbles presentxe2x80x94from the pre-chamber into the main chamber more difficult. Conversely, it is also conceivable to select the temperature in the pre-chamber higher than in the main chamber. This reduces the viscosity of the medium in the pre-chamber, which increases the sealing function of the sealing gap and can effectively prevent penetration of air into the pre-chamber from the outside. The temperature in the main chamber can then be selected independently of the temperature in the pre-chamber in such a way that optimum composition application to the surface is ensured.
Particularly reliable and uniform composition application occurs in the case where the moving surface has recesses, i.e., for example, is in the form of an engraved roll. The term engraved roll is taken to mean a cylindrical roll in which fine grooves have been engraved at regular intervals. However, the recesses can also be punctiform or have any other geometrical shape desired. The profile of the recesses can also adopt any desired forms, for example rectangular or circular.
The apparatus according to the invention is suitable for the application of media of a wide variety of types to the surface, for example polymer melts, solutions of polymers in organic solvents or dispersions of a wide variety of types. Particular advantages arise on use of the apparatus according to the invention for the application of a polymer dispersion to the surface. In contrast to a homogeneous solution, a dispersion is a heterogeneous mixture of a liquid dispersion medium and a dispersed substance finely distributed therein. Of particular practical importance here are dispersions in which the dispersion medium is water and the dispersed substance consists of polymer particles. A dispersion of this type is also referred to as an aqueous polymer dispersion. Aqueous polymer dispersions are widely used as adhesives in the production of labels.
The apparatus according to the invention is particularly suitable for the application of a contact adhesive dispersion to the surface. The term contact adhesive dispersion is taken to mean a dispersion which comprises a pressure-sensitive and self-adhesive polymer whose film formation temperature is below room temperature. The film formation temperature is the temperature at which the dispersed particles melt together to form a transparent, crack-free film. A low film formation temperature can be achieved if a soft polymer, i.e. a polymer having a low glass transition temperature, or a hard polymer to which a plasticizer has been added as additive is used. Polymers based on acrylates and/or methacrylates dispersed in water as dispersion medium are widely used here.